The present invention relates to a dual-shaft machine including a housing having two pistons rotating within the housing without contacting the housing, and a method of producing the dual-shaft machine.
A dual-shaft machine of this type may be, for example, a Roots pump in which two symmetrically configured pistons rotate within a housing without contacting each other or the housing. The two pistons have essentially a figure-eight-shaped cross section and are synchronized by means of a gear drive. Roots pumps of this type are used as delivery pumps in a vacuum or above the atmospheric pressure range. Other dual-shaft machines are Northey pumps, screw compressors, etc.
Due to the contact-free arrangement of the pistons in the housing, return flow of the displaced medium cannot be avoided. The volumetric efficiency of dual-shaft machines of this type is therefore defined by the ratio of the effectively displaced quantity of gas to the quantity of gas that is theoretically possible to displace. The less play there is between the pistons themselves and between the pistons and the housing wall, the less return flow occurs, and consequently the higher the volumetric efficiency of the dual-shaft machine. However, because of the thermal expansion and contraction of the pistons and housing, it is not possible to select the play as low as desired. The dual-shaft machine heats up during operation and expands, thereby reducing the existing play to a point where the danger exists that the pistons will contact each other or the housing.
With respect to the housing, the heat may be dissipated by way of water or air cooling of the outside of the housing. However, the removal of heat from the rotating pistons is effected essentially by the pumped medium itself which either transfers the heat of the piston to the housing or carries it away. Since, during operation of the dual-shaft machine in a vacuum, only a small amount of medium is available to remove the heat, the thermal problems incurred during this use are particularly critical. Since the degree of heating is directly proportionate to the pressure difference between the outlet and the intake of the machine, and a predetermined temperature difference between the pistons and the housing must not be exceeded, a certain pressure difference must be maintained in the operation of the dual-shaft machine in order to avoid contact between the pistons and the housing. Therefore, if rotary pistons are used in a vacuum, the difference between outlet pressure and intake pressure must not exceed a given permissible value, unless special piston cooling measures have been taken.
To permit the highest possible pressure differences for use in a vacuum, it is known to select the play of the machine in the cold state to be particularly large. As the temperature increases, the pistons expand and the play between them, and between the pistons and the inner wall of the pump chamber decreases so that the machine attains its highest volumetric efficiency only when it reaches the preselected operating temperature.
The only difference between dual-shaft machines for use above atmospheric pressure and machines for use in a vacuum is in the cold play between the rotors themselves and between the rotors and the housing. The piston profile for each is essentially the same. For example, in a Roots blower having a pumping capacity of 1000 m.sup.3 /h and intended to be used above atmospheric pressure, the play between the piston and the inner wall of the pump chamber is about 50.mu.. A Roots pump having the same pumping capacity, and intended for use in a vacuum, has a cold play which is greater by about a factor of four. Therefore, dual-shaft machines of the same type and the same order of magnitude require different pistons, depending on their intended use, resulting in high overall manufacturing costs of the machines.
Dual-shaft machines, particularly Roots pumps, have found wide acceptance in many applications since they can be manufactured and operated relatively economically with respect to their pumping capacity. These applications also include the pumping of gases charged with moisture or other, often corrosive additives. Due to these additives, reactions may occur in the region of the piston surfaces or the inner walls of the pump chamber. The products of these reactions, such as rust or the like, become loose and lead not only to impurities in the pumped gases but also to premature wear of the pump.